Radio transmitter



March l2, 19:0. LEV/Ns '2,192,967

RADIO TRANSMITTER Filed Nov. 17, 1936 Gtfomeg Patented Mu. 12, i940UNITI-:D sTATr-:s

ausser RADIO 'Imm r.num lining,v odin, N. J., mmm u naal CorporationotAmerica, a corporation of Deleware Application November 17, 193e', No.111.183 v a am. (ci. ive-111.5)

My invention relatesito improvements in radio transmitters, and morespecicallyto a method of and means for efliciently modulating a radiofrequency carrier over a wide band of frequencies.

i with the signal or modulation current.

while the neising system of medication is en-V tirely satisfactory overa limited range of modulation frequencies, the constant current reactor.

becomes very inefficient when currents having a frequency range from afew cycles per second to several hundreds of thousands cycles per secondare applied. The modulation frequencies employed in high delitytelevision transmission often cover a frequency range of a million ormore cycles per second, and when Vsuch modulation frequencies areapplied in the VHeising modulation system, the losses become excessive.The present invention contemplates a modulation system capable ofelciently handling large amounts ofrpower at modulation frequencies ofthe order of a million or more cycles per second.

An object of my invention is to provide improvide means for modulating aradioffrequency carrier by signal currents covering `a wide range offrequencies.

Another object is to provide improved means for applying signal currentsto modulate an auxiliary carrier, rectifying the modulated auxiliarycarrier currents, filtering the carrier currents,

and applying the rectified currents, which are Vpulsating at signalmodulation frequencies, to the anode circuit of a modulator which iscoupled to the main carrier frequency generator.

A further object is to provide a radio transmitter with means formodulatingthe auxiliary carrier, means for attenuating -currents of oneside band. and forV passing currents of the auxiliary carrier and theother side bond, the latter currents, after being rectified and after lftering the auxiliary carrier, becoming the source of anode currents of amodulator which is coupled to the main carrier frequency generator.

A still further object is toprovlde improved means for modulating acarrier current, radi?" ating the modulated carrier, and suppressing thecarrier during periods of no modulation..

My invention may be best understood by lreference to the accompanyingdrawing, in which 5 Figure 1 is a schematic circuit diagramr of oneembodiment of my invention, and e Figure 2 is a circuit diagramillustrating a ,modification of the circuit of Fig. 1.

Referring to Fig. 1, a. source of signal-fre- 10 quency current isrepresentedby reference numeral I-. 'Ihe signal frequency source I isconnected to the resistance coupled amplifier 3, whose output circuitincludes an anode current source 5'and a resistor 1. A connection ismade l5 from a .point intermediate the ends of the' resistor 'I to atuned circuit 9 which is connected to the anodes of a pair of push-pullmodulator tubes II, I3 which are preferably biased for Class Coperation. The input circuit of the modulator ,20 tubes Il, I3 isconnected to a source I5 of carrier frequency currents. The currentsfrom source `I5 will hereinafter be called the auxiliary carriertodistinguish from the normal carrier which will be described below. ByWay of ex- 2o ample, the auxiliary carrier may be operated at afrequency of seven megacycles.

'I'he tuned circuit 9 in the anode circuits of the push-pull tubes Il,I3 is suitably coupled to a second tuned circuit I1. Both tuned circuits80 l, I1 are resonant to the frequency of the auxiliary carrier, and themutual coupling M is adjusted to give the, desired characteristics tothereby pass the modulated auxiliary carrierA currents withoutattenuating the signal frequen- 35 cy components. A' point I9 on thetuned circuit I'I is grounded. The terminals of the tuned circuit Il areconnected to a full wave rectifier 2l.

The output of therectifier 2| includes a filter which comprises aninductor 23 and a capacitor 40 25. The lter 23, 25 is adjusted to removethe auxiliary carrier ripple and preferably has a time constantof theorder of the period of the aux- Claas `Cv operation and are, connectedto a gener- A4;

ator'35 of carrier frequency currents. The timed anode circuit 29 issuitably coupled by mutual inductance M, or the like, to anantennasystem 31.

The operation of the foregoing system is essentially as follows: Thesignal frequency currents, which may be derived from any source, such asa television scanning system, are amplified by the thermionic tube 3.The amplified currents vary the potential applied, through 'the tunedcircuit 9, tothe anodes of the push-pull,

modulator II, I3 The auxiliary frequency carrier currents are impressedon the grid or input circuits of the modulators I I, I3, andsubsequently transferred to the anode circuits of the modulators. Thusthe currents of Yauxiliary carrier frequency are modulator by the signalfrequency currents in' accordance Awith the conventional theory of anodecircuit modulation.

The modulated auxiliaryc'arrier currents are transferred to the mutuallycoupled circuit I1, whereY the rectier 2| recties the modulatedauxiliary currentsfand thereby produces pulsating unidirectionalcurrents of twice the signal frequency. The pulsating currents areimpressed on the lter 23, 25, which has a time constant of the propercharacteristic to smooth lout the pulsating currents and deliver asteady current upon which is superimposed the original signal frequencyYcurrent.` These steady and varying currents become not only the sourceof anode current for the push-pull modulators 3|, 33, but also th'emodulation current for these tubes 3|, 33. The currents from the maincarrier frequency generator are impressed on the input circuits of themodulator tubes 3l, 33.

The main carrier frequency currents are modulated by the rectiedfiltered currents, when both carrier and ltered currents are present inthe anode circuits of the push-pull modulators 3l, 33. The thusmodulated carrier frequency currents are transferred from the resonantanode circuit 29, through the mutual coupling M, to the antenna circuit31. Since the anode current forpthe modulators 3l, 33 is-a steadycurrent which is varied by the signal frequency currents. it will beapparent that amplitude of the main carrier currentkwill vary inproportion to the signal when signals are impressed on the source I inaccordance with the foregoing description.

The above described method of modulation may be applied to a single sideband transmitter, as indicated by the circuit diagram of Fig. 2. Thesignal frequency source 4I is connected to the input 'of an amplifier 43which has an anode circuit comprising a source of anode cuirent anda'iesistorrv41. The outputA of the amplifier 43 is impressed on aresonant circuit 49, which is connected to the anodes of push-pullmodulator tubes 5I, 53. The control grids of the modulator tubes arepreferably biased for Class C operation 'and are connected totheauxiliary carrier frequency generator 55.

The resonant circuit 49 is vmutually coupled to a second resonantcircuit 51. The terminals of the second resonant circuit may beconnected to the input of a band pass lter 59 through a balancedmodulator 50, which may be omitted in accordance with a descriptionwhich appears Ibelow. The output of the iilter 59 includes a thirdresonant circuit 6I which is mutually coupled to K av fourth resonantcircuit 53. The fourth resonant circuit 63 is connected to a full-waverectifier 65, 61. Y The output of the rectiiers 55, 61 is connected to alter 89, I, which is in turn connectedtothe center tap of a push-pulltransformer 13. The terminals of the push-pull transformer 13, which ispreferably resonantto currents of the main carrier frequency, areconnected to the anodes of a pair of push-pull thermionic modulators 15,11. The input circuits of these tubes 15, 11 are preferably biased forClass C operation and are connected to the source.19 of the maincarriencurrents.

The push-pull transformer 13 is mutually coupled M to a second resonantpush-pull transformer 9|, which is connected to the input circuit of apower amplifier 83. The power amplifier is biased for Class B operationand comprised of suitable tubes 95, 81 which, if triodes, may beneutralized by capacitors 89, 9|. Screen grid tubes may be used in placeof neutralized triodes 95, 81. 'I'he anode circuit of power amplifier 83is comprised of a tuned circuit 93, which is mutually coupled M to anantenna circuit 95.

The `operation of the circuit of Fig. 2 is not greatly different fromthe circuit of Fig. 1. In the latter circuit both the auxiliary carrierand side band lcurrents are impressed on the rectiiier. In the presentcircuit (Fig. 2) the input to the balanced modulator includes two 7megacycle voltages, one of which may be amplitude modulated. In theabsence of modulation the two 7 megacycle voltages combine to produce aresultant 14 megacycle current which is not used since it is attenuatedby the band pass filter 59. The relative amplitudes of the two 7megacycle voltages are adjusted to reduce to a desired amplitude the 'Imegacycle auxiliary carrier in the balanced modulator output.

When signal frequency modulation containing an alternating and a.varying direct current component is applied, two operations are First,the average amplitude of the 7 megacycle carrier which is applied to thebalanced modulator input through transformer 51 takes a value which isdetermined by the direct modulation component, so that the amplitude of'1 megacycle current in the modulator output takes a correspondingvalue; and, second, modulation frequency components are superimposed onthe carrier output of the modulator corresponding to the alternatingcomponents of the signal frequency.

The band pass filter 59 is adjusted to attenuate currents of one sideband, and to pass carrier and the other side band currents. Thesecurrents are rectified, and tre ltered to remove, if necessary, anyresidual carrier ripple. The thus rectied and ltered currents comprisetwo components. There is a d-c component derived from the rectified andfiltered 'I megacycle carrier, the amplitude of whichis proportional tothe average carrier amplitude, and there is also a signal frequencycomponent corresponding to the original signal frequency. The outputcurrents are suillciently amplified to form the anode current source forthe modulators 15. 11. The varying anode currents will transfer the maincarrier currents in varying amplitudes, through the power amplier 83, tothe antenna circuit 95 from which modulated waves will be radiated.During periods of no signal, the amplitude of the radiated carrier willbe determined by the value of the bias applied to the output tubes bythe battery connected to the mid-point of transformer 9|.

The purpose of the balanced modulator is to reduce the amplitudqof boththe main and the auxiliary carrier during periods of zero modulation.'I'his produces an appreciable saving in the transmitter powerconsumptionV during stand by periods. When the auxiliary carrier is soreducedv by the balanced modulator the radiated carrier is likewisereduced since only a carrier of reduced amplitudeand no side bands areimpressed on the rectiiiers 65 and 61, and the radio frequency amplifiertubes- 15 and 11 are operated at reduced power.

In some installations, it may be desirable to l radiate a carrier ofconstant average amplitude by omitting the balanced modulator 60 or thelike, `between the auxiliary modulators 53 and the rectifiers 65, 61.The nally modulated carriers may be transmitted by radiation, byconcentric lines, or any suitable transmission system.v It should beunderstood that half-wave rectiers may be used in place of full-waverectiers. The amount of ltering, between the rectiers of the auxiliarycarrier wave and the main modulators, is determined by the permissibleamount of ripple. In general, the time constant of the filter isapproximately equal to the time of one cycle of the auxiliary carrier ifa half-wave rectier is used, and to half the time of one cycle if afullwave rectifier is employed.

While plate circuit modulation has been used with Class C modulators, byway of illustration, it will be'obvious to those skilled in the art thatmy invention may beapplied to grid modulation. If grid modulation isvcontemplated, I prefer to bias the modulators for Class B modulation.The

"so-called Class B and Class C modulation or amplilication systems arewell known to those skilled in the art and require no detaileddescriptions.

I claim as my invention: 1. In combination, a source of signal frequencycurrents, a source of auxiliary carrier frequency currents, means formodulating the peak amplitudeofsaid auxiliary currents in accordancewith variations of said signal frequency currents and for simultaneouslymodulating the average amplitude of said auxiliary frequencyA currentsin proportion to the average amplitudeof said signal frequency currents,means including a balanced modulator coupled to the output of said firstmentioned` modulating means for reducing the amplitude of said auxiliarycarrier during periods of Zero modulation, means coupled to the outputof said balanced modulator for producing a rectied current having adirect current component proportional to the average amplitude of saidmodulated auxiliary carrier and an alternating current componentproportional to changes in the peak amplitude of said auxiliary carrier,a source of main carrier frequency currents, and means `for modulatingsaid main carrier frequency cur-A rents in accordance with bothcomponents of said rectified current.

2. In combination, a source of signal frequency currents, a source ofauxiliary carrier frequency currents, means for modulating the peakamplitude of said auxiliary currents in accordance with variations ofsaid signal frequency currents and for simultaneously modulating theaverage amplitude of said auxiliary frequency currents in directproportion to the average amplitude of said signal frequency currents,means including a v balanced modulator coupled to the output of saidfirst mentioned modulating means for reducing the amplitude of saidauxiliary carrier during periods of zero modulation, a rectiiiercoupledA to the output of said balanced modulator, a filter networkconnected with said rectifier for smoothing out amplitude variationsoccurring at

